Hi Bill,
A classic method photographers use in photographing invertebrates, from
insects and spiders to flatworms and annelids (to slow them down, or keep them
from moving while the shot is focused), is to put them in the refrigerator for
a while prior to the photo shoot.  Then they are removed from the fridge,
quickly placed in front of the camera (which is already set up of course, and
focused on the general area where the specimen will be).  Then turn on the
lights, focus and shoot before they warm up and become motile again.  Of
course, snails might withdraw into their shells when chilled, I'm not sure,
but it might be worth a try.
Paul M.
Rhode Island